Process for working up of dilute aqueous hydrazine solutions



A ril 3, 1962 E. RAHLFS ETAL 3,028,316 PROCESS FOR WORKING UP 0F DILUTE AQUEOUS HYDRAZINE SOLUTIONS Filed July 28, 1958 INVENTORS:

d- ATTORNEY s nitcd States Patent 9 3,028,316 PROCESS FOR WORKING UP F DILUTE AQUEOUS HYDRAZINE SOLUTIONS- Erich Rahlfs, Leverkusen-Bayerwerk, Werner Thraum, Leverkusen-Schlehusch, and Gunter Henrich, Opladen, Germany, assignors to Farbenfabriken Bayer Aktiengesellschaft, Leverkusen, Germany, a corporation of Germany Filed July 28, 1958, Ser. No. 751,543 Claims priority, application Germany July 31, 1957 8 Claims. (Cl. 202-57) The present invention relates to a process for Working up of dilute aqueous hydrazine solutions.

The formation of hydrazine hydrate from dilute aqueous solutions such as are obtained for example by Raschig synthesis is very complicated and, in addition, involves considerable expense. In order to reduce the cost, it would be possible to make use of the fact that, by distilling aqueous hydrazine solutions in the presence of suitable quantities of acetone, it is possible to obtain distillate fractions the hydrazine content of which is greater than that of the undistilled solution. However in using the conventional, distillation methods, difiiculties arise in the sense that, it is true that while a distillate with a high hydrazine content is obtained initially, the acetone concentration of the body of undistilled liquid or sump is reduced more rapidly than its hydrazine concentration, whereby a large part of the hydrazine is left. The continuous addition of acetone to the undistilled liquid or sump throughout the distillation is ineffective to avoid the depletion of the acetone concentration and permit the recovery in a satisfactory manner of distillate fractions, the hydrazine content of which is in the desired range.

The present invention is concerned with a process for the preparation of hydrazine hydrate from dilute aqueous solution by distillation with acetone, which process consists in that the components, ie the aqueous hydrazine solution and the acetone, are held for a finite time (approximately minutes) in homogeneous solution in the liquid state prior to distillation, and the addition of acetone and the holding step are repeated in stages during the process as the acetone content of the solution becomes too greatly reduced.

It has proved advisable to maintain the molar ratio of acetone to hydrazine between the values of 2.2 and 0.8. in accordance therewith the solution is always adjusted to the ratio of 2.2 before starting each distillation stage and each stage is continued until the value of 0.8 is reached.

The invention is illustrated by the following example without being restricted thereto.

Example The process can be carried out substantially as follows using an apparatus as shown in the accompanying drawing. The charging vessel 1 contains 2% hydrazine hydrate solution containing 50 g./l. of acetone, this solution being held in the vessel in a homogeneous condition for at least 10 minutes before commencing operations. The solution is supplied continuously at a rate of 34 litres per hour to the upper fractionating column 2. The holding vessels 3a and 3b, which are provided so that the solution may remain therein for a period of time, are constantly charged with acetone from the storage vessels 7a and 7b provided for this purpose, the vessel 3a being charged at the rate of 20 g. per hour and the vessel 3b at the rate of 7 g. per hour of acetone. Such an amount of heat is supplied to the sump flask 6 by gas flame heating that about 500-600 cc. of distillate per hour-cooled in cooling device 4-are received in the distillate receiver 5. By suitable adjustment of the cock, liquid is constantly ice withdrawn from the sump flask 6 at such a rate that the flask is always half full.

The sump discharge is practically free from hydrazine and acetone, while these components are found in correspondingly concentrated form in the distillate.

We claim:

1. Process for working up dilute aqueous hydrazine solutions by distillation with acetone, which comprises the steps of maintaining the aqueous hydrazine solution and the acetone in homogeneous solution in the liquid state until a substantial amount of a hydrazine-acetone compound is formed, thereafter in a vertical distilling zone distilling off the said hydrazine-acetone compound, adding acetone to the undistilled liquid during the distilling at a point along the zone, and repeating the steps of maintaining the aqueous hydrazine solution and the acetone in homogeneous solution in the liquid state until a substantial amount of a hydrazine-acetone compound is formed, distilling ott' the said hydrazine-acetone compound and adding acetone to the undistilled liquid at least one more time at a second point along the zone.

2. Process according to claim 1, which comprises adjusting the molar ratio of the acetone to the hydrazine to 2.2 prior to the distillation step and re-adjusting to this value during the distillation any time the said molar ratio drops to below 0.8.

3. Process for working up dilute aqueous hydrazine solutions by distillation with acetone, which comprises the steps of maintaining the aqueous hydrazine solution and the acetone in homogeneous solution in the liquid state for about 10 minutes until a substantial amount of a hydrazine-acetone compound is formed, thereafter in a vertical distilling zone distilling off the said hydrazineacetone compound, adding acetone to the undistilled liquid during the distilling at a point along the zone, and repeating the steps of maintaining the aqueous hydrazine solution and the acetone in homogeneous solution in the liquid state until a substantial amount of a hydrazineacetone compound is formed, distilling off the said hydrazine-acetone compound and adding acetone to the undistilled liquid at least one more time at a second point along the zone.

4. In the process for working up dilute aqueous hydrazine solutions by distillation thereof in the presence of acetone, the improvement which comprises holding a mixture of aqueous hydrazine solution and acetone for a predetermined period of time in homogeneous solution in the liquid state prior to distillation thereof, passing said mixture of aqueous hydrazine solution and acetone into the upper end portion of a distillation zone, distilling off hydrazine and acetone, adding acetone to the distillation zone at a multiple number of points spaced vertically along said zone to thereby form a series of liquid pockets at said points, holding the aqueous hydrazine solution and the acetone in homogeneous solution in the liquid state at each of said points, distilling off acetone and hydrazine following said holding, and collecting a hydrazine enriched distillate and a hydrazine poor fraction from the sump for said distillation zone and maintaining the volume in said sump substantially constant throughout said distillation.

5. Improvement according to claim 4 wherein the time of holding said aqueous hydrazine solution and said acetone prior to distillation is about ten minutes.

6. improvement according to claim 4 wherein said mixture of acetone and hydrazine is continuously supplied to the upper end portion of said distillation zone.

7. Improvement according to claim 4 wherein the addition of acetone is greater at the points nearer the upper end portion of the distillation zone and a lesser amount of acetone is added at the points positioned lower nearer the sump for the distillation zone.

8. Improvement according to claim 4, wherein the molar ratio of the acetone to the hydrazine is adjusted prior to distillation to 2.2 and readjustment to this value takes place after the ratio has fallen below 0.8 during the distillation.

References Cited in the file of this patent UNITED STATES PATENTS 2,537,791 Schwartz Jan. 9, 1951 Bircher Dec. 28, 1954 Miller Ian. 31, 1956 OTHER REFERENCES Audrieth: The Chemistry of Hydrazine, 1951, pp. 4344. 

1. PROCESS FOR WORKING UP DILUTE AQUEOUS HYDRAZINE SOLUTIONS BY DISTILLATION WITH ACETONE, WHICH COMPRISES THE STEPS OF MAINTAINING THE AQUEOUS HYDRAZINE SOLUTION AND THE ACETONE IN HOMOGENEOUS SOLUTION IN THE LIQUID STATE UNTIL A SUBSTANTIAL AMOUNT OF A HYDRAZINE-ACETONE COMPOUND IS FORMED, THEREAFTER IN A VERTICAL DISTILLING ZONE DISTILLING OFF THE SAID HYDRAZINE-ACETONE COMPOUND, ADDING ACETONE TO THE UNDISTILLED LIQUID THE DISTILLING AT A POINT ALONG THE ZONE, AND REPEATING THE STEPS OF MAINTAINING THE AQUEOUS HYDRAZING SOLUTION AND THE ACETONE IN HOMOGENEOUS SOLUTION IN THE LIQUID STATE UNTIL A SUBSTANTIAL AMOUNT OFF THE SAID HYDRAZINE-ACETONE COMPOUND IS FORMED, DISTILLING OFF THE SAID HYDRAZINE-ACETONE COMPOUND AND ADDING ACETONE TO THE UNDISTILLED LIQUID AT LEAST ONE MORE TIME AT A SECOND POINT ALONG THE ZONE. 